1. Field of the Invention
The present invention relates to an exhaust system for a small watercraft. More particularly, the present invention relates to an exhaust system capable of reducing noise of exhaust gas during a low-speed operation of an engine, and a jet-propulsion personal watercraft having the exhaust system.
2. Description of the Related Art
In recent years, jet-propulsion personal watercraft have been widely used in leisure, sport, rescue activities, and the like. The personal watercraft is equipped with an engine mounted within a space surrounded by a hull and a deck. The engine is configured to drive a water jet pump, which pressurizes and accelerates water sucked from a water intake generally provided on a bottom surface of the hull and ejects it rearward from an outlet port of the water jet pump. As the resulting reaction, the personal watercraft is propelled forward.
Japanese Patent No. 3290037 discloses that an exhaust gas from an engine is discharged outside a watercraft through an exhaust system comprising an exhaust manifold, a muffler, pipes connecting these, and the like. In an exhaust system mounted in some personal watercraft, water is supplied to an exhaust gas flowing inside to reduce an energy of the exhaust gas, thereby reducing noise of the exhaust gas.
The exhaust system configured to supply water to the exhaust gas is comprised of an exhaust manifold connected to an exhaust port of an engine, a first exhaust pipe connected to a downstream side of the exhaust manifold, a water muffler connected to a downstream side of the first exhaust pipe and having a predetermined volume, a second exhaust pipe configured to allow the water muffler to communicate with the outside of the watercraft, and the like. In general, the first exhaust pipe connecting the exhaust manifold to the water muffler is provided with a water-supply portion configured to supply water to the exhaust gas flowing inside. In this construction, while the exhaust gas from the engine flows within the exhaust manifold, the first exhaust pipe, the water muffler, and the second exhaust pipe, the exhaust gas is cooled by the water supplied from the water-supply portion to allow the energy to be reduced, and is thereafter discharged outside.
Internal state of the water muffler varies depending on an engine speed. For example, since a high-temperature and large-volume exhaust gas flows within the water muffler at a high speed during a high engine speed, the water supplied to an inside of the exhaust system is diffused and becomes mist. During this state, the energy of the exhaust gas is reduced most, and noise of the exhaust gas is correspondingly reduced most. On the other hand, since the exhaust gas flows with a relatively low speed during a low engine speed, the water supplied to the exhaust system tends to drop to and remain in an inner bottom portion of the water muffler.
In order to discharge the water remaining in the inner bottom portion of the water muffler outside the watercraft by using the exhaust gas, the second exhaust pipe configured to allow the water muffler to communicate with the outside of the watercraft, is provided such that its upstream end in a flow passage of the exhaust gas protrudes and opens within the water muffler. In this construction, if the water reaches a vicinity of the upstream end of the second exhaust pipe in the inner bottom portion of the water muffler, part of the water is pushed from an opening of the upstream end of the second exhaust pipe outside the watercraft through the second exhaust pipe by the exhaust gas flowing from the water muffler to the second exhaust pipe. Since the exhaust gas is discharged together with the water within the water muffler, the exhaust noise being emitted outside the watercraft is reduced. The upstream end of the second exhaust pipe is located above to be sufficiently spaced apart from a inner bottom surface of the water muffler, in order to reduce a back pressure in the muffler.
However, shaking the body of the watercraft agitates the water within the water muffler and causes turbulence therein. The turbulent water within the water muffler opens and closes the opening of the upstream end of the second exhaust pipe protruding into the water muffler, thereby causing a relatively large noise to be generated. This noise forms part of the exhaust noise emitted outside the watercraft, and makes it difficult to reduce the exhaust noise.
In particular, in the construction in which the upstream end of the second exhaust pipe is spaced relatively apart from the inner bottom surface within the water muffler, a large amount of water remains in the water muffler, and the turbulent water due to shaking of the body increases. Therefore, the exhaust noise caused by opening and closing of the opening of the upstream end increases. In this case, since a sound-pressure level of the exhaust noise is very high in a frequency range which is unpleasant to human beings, it is desirable to minimize this sound-pressure level. In addition, since a substantial volume as an expansion space of the water muffler decreases due to the volume of the water remaining therein, a muffling effect of an exhaust chamber is reduced.
Nonetheless, it is difficult to reduce the exhaust noise unpleasant to the human beings by increasing the volume of the water muffler. Such a condition occurs in small watercraft having a water muffler within an exhaust system therein, other than the personal watercraft.